Normally, the viscosity of a lubricating oil decreases as the temperature rises. For this reason, the viscosity is generally high at low temperatures and low at high temperatures. The types of lubricating oils even differ according to the environment (especially temperature) in which they are used. In the case of lubricating oils used in both low-temperature environments and high-temperature environments, if they are low-viscosity types, loss of oil film may occur because the viscosity is too low at high temperatures, and they may not serve their function as a lubricating oil. On the other hand, if they are high-viscosity types, the viscosity at low temperatures may be too high and churning losses may increase, and pump feeds of oil may not work, giving rise to problems of seizure and wear.
It is important that viscosity is low at activation start-up times (when switching from a stopped state to a working state, that is, at low temperatures). This is because, if viscosity is high at such activation start-up times, an initial starting force is necessary to go from the stopped state to the working state. On the other hand, once the machinery has started working, viscosity becomes irrelevant. If the machinery continues working, the machinery acquires heat, and its temperature rises (for example, to about 100° C.). When a high temperature is reached, there is a possibility, as mentioned above, that the viscosity falls too much and the oil film is broken.
It is difficult to maintain the necessary viscosity in wide-ranging temperature conditions with just a single lubricating oil. WO96/11244 has therefore disclosed a lubricating oil which, by virtue of combining a low-viscosity lubricating oil and a high-viscosity lubricating oil, uses only the characteristics of the low-viscosity lubricating oil at low temperatures and makes use of the feature that viscosity rises by having a high-viscosity lubricating oil mix with the low-viscosity lubricating oil at high temperatures, so that it functions at both low temperatures and high temperatures.
However, in the method described in WO96/11244 there is an issue in that the separation temperature and the kinematic viscosity are undoubtedly determined by the kinds and ratios of the oils combined and it can be hard to produce the characteristics required by the application.